(CNN) -- A 6.8-magnitude earthquake struck Monday off the third-largest island in the Philippines, killing at least 12 people, an official said.

The quake struck about 11:49 a.m. (10:49 p.m. Sunday ET) about 70 kilometers (44 miles) from the coastal city of Dumaguete on the Philippine island of Negros, the U.S. Geological Survey said.

The dead included two elementary school children, authorities said, according to the Philippines News Agency. The girls died when walls at their schools collapsed on them, National Police spokesman Chief Supt. Agrimero Cruz Jr. said.

The Office of Civil Defense put the official death toll at 12, said Maj. Reynaldo Balido, although there were unconfirmed reports of more deaths.